Electrospinning is a versatile method to produce polymer fibers with diameters in the micron, sub-micron and nano (<100 nm) range. Numerous polymeric materials have been electrospun into continuous, uniform fibers, and various applications of the fibers have been widely recognized. The method employs electrostatic forces to stretch a polymer jet and make superfine fibers. Electrohydrodynamic instabilities that occur in electrospinning, charge density of the electrified jet (and indirectly, solution conductivity), surface tension, and viscoelasticity of the solution have been shown to play important roles both in making the production of fibers possible and in controlling the size and uniformity of the fibers. The development of internal structure in such fibers has generally been limited to crystallization of homopolymer or macrophase separation of a polymer blend during the drying and solidification of the fiber, inclusion of immiscible additives such as clays, nanotubes and metallic or oxide particles. Surface structures attributed to “breath figures” have also been shown.
Block copolymers offer an alternative method by which internal structure can be induced in electrospun fibers via microphase separation. In bulk, block copolymers are known to form microphase separated structures such as spheres, cylinders, gyroids and lamellae, depending on molecular weight, volume fractions of components and the degree of immiscibility of the different polymer blocks. In thin films, it has been shown that surface forces and confinement effects are strong enough to alter the phase separation behavior. However, no such information is currently available on microphase separation in a confined cylindrical, sub-micrometer sized and fiber-like geometry. Electrospinning of block copolymers is therefore not only promising for applications involving surface chemistry, drug delivery and multi-functional textiles, but is also of intrinsic scientific interest.
The wetting behavior of a solid surface is important for various commercial applications and depends strongly on both the surface energy or chemistry and the surface roughness. Currently, surfaces with a water contact angle above 150° are considered to be “superhydrophobic” and are the subject of great interest for their water proof and self-cleaning usages. There is a need to develope fiber-forming processes and products that would demonstrate the desired surface characteristics, such as superhydrophobicity, as well as other properties, such as mechanical strength and integrity.